[Cython] [cython-users] Re: How to find out where an AttributeError is ignored

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Fri Jan 27 21:03:30 CET 2012

On 01/27/2012 05:58 PM, Stefan Behnel wrote:
> mark florisson, 27.01.2012 17:30:
>> On 27 January 2012 16:22, mark florisson<markflorisson88 at gmail.com>  wrote:
>>> On 27 January 2012 15:47, Simon King<simon.king at uni-jena.de>  wrote:
>>>> Hi all,
>>>> I am still *very* frustrated about the fact that Cython does not tell
>>>> where the error occurs. Since about one week, I am adding lots and
>>>> lots of lines into Sage that write a log into some file, so that I get
>>>> at least some idea where the error occurs. But still: Even these
>>>> extensive logs do not provide a hint on what exactly is happening.
>>>> How can I patch Cython such that some more information on the location
>>>> of the error is printed? I unpacked Sage's Cython spkg, and did "grep -
>>>> R ignored .", but the code lines containing the word "ignored" did not
>>>> seem to be the lines that are responsible for printing the warning
>>>> message
>>>>    Exception AttributeError: 'PolynomialRing_field_with_category'
>>>> object has no attribute '_modulus' in  ignored
>>>> Can you point me to the file in Sage's Cython spkg which is
>>>> responsible for printing the warning?
>>>> Best regards,
>>>> Simon
>>> These messages are written by PyErr_WriteUnraisable, which is a
>>> CPython C API function that writes unraisable exceptions. There are
>>> typically two reasons for unraisable exceptions:
>>>     1) as Robert mentioned, a function that does not allow propagation
>>> of exceptions, e.g.
>>>         cdef int func():
>>>             raise Exception
>>>         Here there is no way to propagate the raised exception, so
>>> instead one should write something like
>>>             cdef int func() except -1: ...
>>>         Alternatively one may use 'except *' in case there is no error
>>> indicator and Cython should always check, or "except ? -1" which means
>>> "-1 may or may not indicate an error".
>>>     2) in deallocators or finalizers (e.g. __dealloc__ or __del__)
>>> For functions the right thing is to add an except clause, for
>>> finalizers and destructors one could use the traceback module, e.g.
>>>     try:
>>>         ...
>>>     except:
>>>         traceback.print_exc()
>>> If this all still doesn't help, try setting a (deferred) breakpoint on
>>> __Pyx_WriteUnraisable or PyErr_WriteUnraisable.
>> Actually, I don't see why the default is to write unraisable
>> exceptions. Instead Cython could detect that exceptions may propagate
>> and have callers do the check (i.e. make it implicitly "except *").

As for speed, there's optimizations on this, e.g., "except? 32434623" if 
the return type is int, "except? 0xfffff..." if the return type is a 

And for floating point, we could make our own NaN -- that's obscure 
enough that it could probably be made "except 
cython.cython_exception_nan" by default, not "except? 

>> Was this not implemented because Cython only knows whether functions
>> may propagate exceptions at code generation time by looking at the
>> presence of an error label?
>> Maybe it could keep code insertion points around for every call to
>> such a potential function and if the function uses the error label
>> have the caller perform the check? Although I do forsee problems for
>> external such functions... maybe Cython could have it's own
>> threadstate regardless of the GIL which would indicate whether an
>> error has occurred? e.g. CyErr_Occurred()?
> Yep, those are the kind of reasons why writing unraisable exceptions is the
> default.

Still, the need to explicitly declare "except *" keeps coming up again 
and again, and is really a blemish on the usability of Cython. When 
teaching people Cython, then it's really irritating to have to follow 
"all you need to do is add some 'cdef' and some types" with "and then 
you need to remember to say "except *", or you're in deep trouble". 
Cython sort of looks very elegant until that point...

Long-term we should change CPython to make sure that PyErr_Occurred 
doesn't need the GIL :-) (there's really no reason it should need to go 
beyond checking a thread-local variable).

We could also change the Cython ABI/calling convention -- use the return 
code always for reporting error status (unless there's an "except" or it 
is "cdef extern") and report return value in a pointer out-argument. 
That'd generalize to support multiple typed return values as well. Of 
course, there's a lot of downsides to changing ABI...

Dag Sverre

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